1. Field of the Invention
The present invention relates to control of an apparatus for preventing an image blur, which apparatus is used in cameras, optical devices, and the like.
2. Related Background Art
As a conventional image blur prevention apparatus of this type, an apparatus of a type for driving a correction optical system attached in or in front of a photographing lens on the basis of a vibration gyro type angular velocity sensor utilizing a Coriolis' force generated in a vibrating element in a camera-fluctuation detection sensor, or its output (in the case of the angular velocity sensor, an integral circuit for obtaining an angular deviation by integrating the detected angular velocity is normally added) is used. Also, an apparatus for extracting and outputting only a camera-fluctuation amount by a photographer by detecting, e.g., motion vectors of an object image using a 2-dimensional area sensor arranged in a camera main body, and driving the correction optical system on the basis of the output value is disclosed in Japanese Laid-Open Patent Application No. 3-192227.
However, in the conventional image blur prevention apparatuses, for example, in a system utilizing a mechanical sensor such as a vibration gyro, vibration detection performance at the low-frequency side inevitably deteriorates due to the nature of the sensor. For this reason, when a photograph is taken with a long shutter time, and a low-frequency vibration is generated, an image blur prevention effect is difficult to obtain (a photograph to be taken with a short shutter time is not easily influenced by a low-frequency vibration even when no image blur prevention function is in operation).
Although a system utilizing an optical sensor such as an area sensor has high vibration detection performance at the low-frequency side, the calculation speed for image processing of actual image data lags behind a photographing operation at the high-frequency side exceeding 10 Hz, and a fine vibration cannot be sufficiently corrected due to a specific ratio of the number of pixels of the area sensor to a correction range. With either one of these systems, it is difficult to obtain sufficient performance as an image blur prevention apparatus for a camera.
Also, an apparatus, which has both a mechanical sensor and an area sensor, and switches the outputs from the two sensors in correspondence with situations to perform an image blur prevention operation, has been proposed.
For example, Japanese Laid-Open Patent Application No. 4-215623 discloses a technique for selecting one of the outputs from the two sensors to be used in accordance with a shutter time. More specifically, an AF integral time and an actual exposure time are compared with each other, and when the AF integral time is longer than the exposure time, image blur prevention is performed in accordance with the output from the mechanical sensor; otherwise, image blur prevention is performed in accordance with the output from the area sensor.
Japanese Laid-Open Patent Application No. 4-277728 discloses a technique for (1) performing image blur prevention in accordance with the output from the mechanical sensor when an output cannot be obtained from the area sensor (e.g., when an object field is dark or when a mirror-up state is set in a TTL type system), and (2) performing image blur prevention in accordance with the output from the area sensor when the output from the mechanical sensor is small.
In the techniques disclosed in these two references, the sensors are not selectively used in accordance with the nature of a generated camera fluctuation, and each sensor must often detect a camera fluctuation, which is not easily detected by the selected sensor.
Furthermore, Japanese Laid-Open Patent Application No. 4-163533 discloses a technique for selecting the sensor to be used in accordance with the velocity of a generated camera fluctuation. More specifically, when the velocity of the generated camera fluctuation is smaller than a predetermined value, image blur prevention is performed in accordance with the output from the area sensor; otherwise, image blur prevention is performed in accordance with an angular velocity sensor as the mechanical sensor.
However, in the technique for selecting the sensor to be used in accordance with only the camera fluctuation velocity as in the technique disclosed in above-mentioned reference, when a camera fluctuation velocity is low but a high-frequency camera fluctuation is generated, the area sensor is selected. In this case, precise detection is disturbed due to a limitation on the accumulation time of the area sensor.